Gel electrophoresis is commonly used to separate by molecular size biological molecules, such as deoxyribonucleic acid ("DNA"), ribonucleic acid ("RNA") and proteins. To perform gel electrophoresis, a polymeric gel, such as polyacrylamide, is formed in a glass tube, or between spaced glass or plastic plates. The tube or plates are then placed in a container along with anode and cathode elements at the top and bottom of the gel. Sample wells formed in the top of the gel are first filled with buffer solutions. Molecule samples prepared in a sample buffer that may contain a tracking dye are then placed in the wells. Electrophoretic buffer solutions containing conductive ions are added to the container to make electrical contact between the gel, the samples in the wells and the anode and cathode elements. A voltage is then applied across the gel, which causes the sample molecules and any tracking dye to migrate toward the bottom of the gel, and separate into bands whose migration distance depends on molecular size.
Previously known commercial gel electrophoresis systems, such as the XCell II Mini-Cell.TM. manufactured by Novel Experimental Technology, Incorporated, San Diego, Calif. ("NOVEX"), include a container for receiving a first buffer solution and a buffer core assembly that comprises a pair of gel cassettes affixed to front and back sides of a U-shaped buffer core. The space defined by the upraised side members of the buffer core and the end faces of the gel cassettes forms an upper buffer chamber. The buffer core assembly is immersed in the first buffer solution in the container, and a second buffer solution is added to the upper buffer chamber.
For accurate electrophoretic separation, the first and second buffer solutions must be isolated from one another. To provide isolation, prior art electrophoresis systems use various methods to hold the gel cassettes in contact with the buffer core and secure the buffer core assembly in the container. Previously known electrophoresis systems commonly use a buffer core subassembly containing clamps or latches that secure the gel cassettes to the buffer core. Once the cassettes are secured, the buffer core subassembly must then be loaded in the container prior to electrophoretic separation.
For example, the SE 200 Series Mini-Gel System manufactured by Hoefer Pharmacia, San Francisco, Calif., includes a buffer core subassembly that uses four spring clamps to secure a pair of gel cassettes to a buffer core. Similarly, the Mini-PROTEAN II Electrophoresis Cell manufactured by Bio-Rad, Hercules, Calif. ("Bio-Rad"), includes a buffer core subassembly that uses latches to secure gel cassettes to a buffer core. Alternatively, the Ready-Gel Cell manufactured by Bio-Rad, includes a buffer core subassembly having a pair of cams that secure gel cassettes to a buffer core. In each of these prior art systems, the user must first construct a clamping subassembly that is then loaded into the container prior to performing electrophoresis. It would be desirable to provide a clamping device that is easier to use and does not require separate clamping subassemblies.
Other prior art electrophoresis systems avoid the need for separate clamping subassemblies by using the electrophoresis container as part of the clamping mechanism. For example, the XCELL II Mini-Cell.TM. uses two wedge blocks inserted in the electrophoresis container to secure the gel cassettes against the buffer core. The wedge blocks, however, must be carefully inserted to obtain proper clamping action. For example, if the wedge blocks are not properly inserted, they may slip relative to one another, and may release pressure on the gel cassettes and degrade isolation between the first and second buffer solutions. Further, the wedge blocks provide a clamping force that varies depending on the amount of force used to insert the wedge blocks into the container. If too much force is applied, the components of the electrophoresis system may become over-stressed and may eventually fracture. If too little force is applied, the gel cassettes may not be securely held in contact against the buffer core.
An improvement on the foregoing wedge block design is described in Japanese Patent Publication 62-201055 (Application No. 61-088779), in which two fastening screws adjust the pressure applied by the upper wedge block and ensure that the wedge blocks maintain continuous pressure on the gel cassettes and buffer core. Although a stop screw is provided in that device to provide reproducible pressure to the wedge blocks, the stop screw may inadvertently loosen or tighten with repeated use, thus varying the applied force from a predetermined value. Accordingly, if too much force is applied, the components of the electrophoresis system may become over-stressed and fracture. Conversely, if too little force is applied, the gel cassettes may not be securely held in contact against the buffer core.
In view of the problems associated with prior art clamping methods and apparatus, it is desirable to provide for gel electrophoresis systems a single cam-activated clamp that requires no clamping subassembly to reliably secure electrophoresis gel cassettes to a buffer core.
It is further desirable to provide for gel electrophoresis systems a single cam-activated clamp that provides a consistent and reproducible clamping force each time the apparatus is used.
It is also desirable to provide for gel electrophoresis systems a single cam-activated clamp that provides a positive stop to prevent overstressing the electrophoresis system components.